The present invention relates to a heat and oxidation resistive high strength material used in structural body pans subject to bleaching in high temperature/oxidative atmosphere, the body structure of an orbital space plane, a combustor, the combustor for a gas turbine, a blade, and a nozzle; the present invention also relates to a method for producing the heat and oxidation resistive high strength material.
Concerning the fields in energy, material production process, and spacecraft, developments in the heat resistive materials having thermal insulation, heat resistance, and excellent resistance to environmentally induced damages are cited as important technical topics for materials used under extreme environmental circumstances.
As heat resistive materials there are metallic materials, composite materials, and ceramics to name a few. As far as heat resistive alloy is concerned, there is a super alloy which has Ni, Co, and Fe as the constituent bases. However, Ni, the main constituent of a Ni-based super alloy, has a melting point of 1455.degree. C., and therefore, this alloy cannot be used in an environment with temperature range that goes beyond this point. For this reason, an environmentally induced damage insulation layer is formed on the surface of this alloy. As one of the representative examples, a thermal barrier coating (TBC) method is presented which involves spraying melted ceramics of ZrO.sub.2 and Y.sub.2 O.sub.3 into the middle layer of a super alloy, produced with the purposes of relaxing thermal stress, improving adhesion, enhancing oxidation resistance, and improving anti-corrosion at the surface. Furthermore, JPA62-156938 describes a functionally gradient material (FGM) which relaxes thermal stress by having the composition ratio of ZrO.sub.2 and Y.sub.2 O.sub.3 change continuously at the middle layer between the substrate and the ceramics.
There are others such as an intermetallic compound and a high melting point metal. Some of the intermetallic compounds as a heat resistive structure are Al compounds of iron family (Fe, Ni, Co) and Ti, but these compounds leave some room for improvement in terms of hardness, workability, and resistive to oxidation. On the other hand, the high melting point metals of W, Mo, Nb, Ta etc., have high thermal conduction and has good resistance against heat but has a weakness of being abraded easily by oxidation; and therefore, there is a need to develop an alloy having strength and resistance to oxidation or a surface process which imparts these characteristics.
Moreover, there are composite materials which have heat resistancy and high strength. Composite materials having high strength fibers at a high temperature range improve the strength of matrix materials at a high temperature range. In terms of matrix types there are fiber reinforced plastics (FRP), fiber reinforced metals (FRM), fiber reinforced ceramics (FRC), and carbon fiber reinforced carbons (CFRC). Limit on the highest usable temperature depends on the type of matrix used. For the plastic type, the temperature is 300.degree. C.; for the metal type, 1300.degree. C.; for the ceramic type, 1800.degree. C.; and for the carbon type, 3000.degree. C. approximately. Specifically, the density of the CFRC is less than 2.0 and the strength of this material does not deteriorate until over 300.degree. C.; the material is known as a super heat resistive material for retaining excellent strength and excellent hardness at high temperatures. However, since the CFRC is made of carbon only, in the oxidative atmosphere of around 500.degree. C., oxidative abrasion becomes noticeable. Therefore, this material cannot be used at temperatures above 500.degree. C. in an environment of oxidative atmosphere. Therefore, in order to use the CFRC even in such an environment, an anti-oxidation treatment becomes necessary. That is, the determining factor of the usable heat resistive temperature of the CFRC in the oxidative atmosphere is the durability of the anti-oxidation treatment. An example of the anti-oxidation treatment is an anti-oxidation coating formed on the substrate surface.
One of the main coating elements of the anti-oxidation coating is SiC. For forming SiC, there are a chemical vapor reaction (CVR) method or a chemical vapor deposition (CVD) method. The CVR method involves diffusing metallic silicon vapor into the substrate and reacting with the carbons in the substrate to form SiC. Passages are required for producing this reaction, and because holes are very difficult to get rid of, the coating is left with many holes. In the oxidative atmosphere, oxygen enter into the substrate through these holes and cause damages, and therefore, there is the oxidation problem. Since the CVD method involves forming a coating by depositing a coating at the atomic level, SiC with very high purity and a very fine crystal structure can be formed. However, since the expansion coefficient of the CFRC is small (0 to 1.times.10.sup.-6 /.degree.C.), microcracks form across the thickness of the coating by thermal stress caused by the differences in thermal expansion. Through these microcracks, oxygen of the oxidative atmosphere enter to cause damages to the substrate. Therefore, microcracks must be sealed, and this is done by tetraethyl orthosilicate (TEOS), for example, which impregnates into the microcracks of SiC to seal it; this process is described in Spacecraft Technical Research Report TR-946, "Trial Product Test of Carbon Composite Combustor for Low Thrust Storage Propulsion Chemical Engine" (October, 1987), or The Third Symposium for the Advances in Environmentally Induced Damage Super Resistant Materials, "Petroleum Pitch Type C/C Composite Material Composite Formation and Anti-Oxidation Technology" (October, 1987).
For an anti-oxidation coating of greater temperature range (2000.degree. C.), two layer coatings of Ir and Al.sub.2 O.sub.3, placed on top of Ir, are formed on the C/C material surface by a sputtering deposition instrument. However, this coating is subject to cracks when the temperature is made to rise to and fall from the high temperature range, and therefore, a sealing process is required (described in The Third Symposium for the Advances in Environmentally Induced Damage Super Resistant Materials, "Petroleum Pitch Type C/C Composite Material Composite Formation and Anti-Oxidation Technology" (October, 1987)). Therefore, it is necessary to examine the sealing materials and the process methods for maintaining anti-oxidation property in the 2000.degree. C. range.
On the other hand, as a method of other formation of the anti-oxidation surface of the C/C material, a material of metallic layers consisting of a hafnium, tantalum or zirconium foil between the rhenium or silicon carbite layers is described in JPA1-23048 No. 7 of "Heat and Oxidation Resistive Reinforced Material and its Production Method." The anti-oxidation coating layer of this materials reacts, in the oxidative atmosphere at 2000.degree. C., under certain combination of materials to produce products that possibly lower the anti-oxidation property. Also, high temperature anti-resistive carbon materials having a silicon carbite film formed on top of the carbon substrate, and Hf and Zr metal films formed on top of the silicon carbite film, and an Ir film formed on top of this is disclosed in JPA4-149083.
According to the known technologies of the above, it would be difficult to obtain light and strong materials in the 2000.degree. C. range. That is, it became evident that a carbon fiber reinforced carbon as a highly strong material does not have adequate heat and oxidation resistivity in the oxidative atmosphere at the 2000.degree. C. range. SiC, which is used often as a heat and oxidation resistive coating layer formed on the substrate surface, is oxidized to SiO.sub.2 in the high temperature range, and higher the temperature, more of this product forms. Additionally, as heating and cooling is repeated, the oxidized coating of SiO.sub.2 is subject to peeling and therefore, durability cannot be achieved. Furthermore, if the temperature goes above 1700.degree. C., SiO.sub.2 melts, and in the case of materials on the body of a spacecraft which receives bombardment on the surface, the melted material scatters and the abrasion becomes noticeable. Therefore, the limit on the usable temperature of SiC as a heat and oxidation resistive coating layer is 1700.degree. C.
Additionally, as an easily accessible technology, there is a plasma flame coating method for forming high melting point ceramics. By this method, even if a high melting point ceramics layer or a metal layer is formed on the carbon fiber reinforced carbon substrate, there are cases in which cracks or peeling occurs after the formation of the layer or when the layer receives thermal shocks because of the differences in the expansion coefficients between the layer and the substrate or a lack of good adhesion between the two. In these kinds of situation, the substrate oxidizes and is subject to abrasion, and the layer ceases to function as a good heat and oxidation resistive coating layer.
The purpose of the present invention is to solve the aforementioned problems of the prior arts by presenting, first, a heat and oxidation resistive high strength material having a heat and oxidation resistive coating layer that has anti-thermal shock, anti-corrosion, and anti-oxidation properties along with excellent adhesive property to the surface of a heat resistive high strength substrate made of carbon and, second, a production method thereof.